A preliminary study into the use of tree-ring and foliar geochemistry as bio-indicators for vehicular NO_x pollution in Malta

ABSTRACT

Emissions from traffic over the past few decades have become a significant source of air pollution. Among the pollutants emitted are nitrogen oxides (NO_x), exposure to which can be detrimental to public health. Recent studies have shown that nitrogen (N) stable isotope ratios in tree-rings and foliage express a fingerprint of their major N source, making them appropriate for bio-monitoring purposes. In this study, we have applied this proxy to Aleppo pines (Pinus halepensis) at three distances from one of the busiest roads in Malta, a country known to suffer from intense traffic pollution. Our results showed that N and organic carbon (C) stable isotope ratios in tree-rings do not vary over the period 1980–2018 at any of the investigated sites; however, statistically significant spatial trends were apparent in both tree-rings and foliage. The roadside and transitional sites exhibited more positive δ¹⁵N and more negative δ¹³C values compared to those at a rural control site. This is likely due to the incorporation of ¹⁵N-enriched NO_x and ¹³C-depleted CO₂ from traffic pollution. Sampled top-soil also exhibited the δ¹⁵N trend. Our results constitute the first known application of dendrogeochemistry to atmospheric pollution monitoring in Malta.

KEYWORDS:

• Carbon-13
• dendrogeochemistry
• foliage
• isotope ecology
• Malta
• motor vehicles
• nitrogen-15
• NO_x
• traffic
• tree-rings

Acknowledgements

The authors would like to thank Vincent M. Mifsud for his assistance in sample collection. Furthermore, we would like to thank Ian Washbourne and James Weir at the University of Stirling for the loan of their Retsch MM-200 ball mill.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

Duncan V. Mifsud is the grateful recipient of an Endeavour Scholarship (Republic of Malta). The Endeavour Scholarship Scheme is part-financed by the European Union – European Social Fund (ESF): Operational Programme II – Cohesion Policy 2014-20.